1. Field of the Invention
This invention relates generally to a powertrain for a motor vehicle, particularly to the hydraulic control circuit of powershift transmission located in the powertrain.
2. Description of the Prior Art
A dual clutch transmission (DCT), also called a powershift transmission, is a geared mechanism employing two input clutches used to produce multiple gear ratios in forward drive and reverse drive. It transmits power continuously using synchronized clutch-to-clutch shifts.
The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with certain gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with the other gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
A powershift transmission launches the vehicle from a stopped or nearly stopped condition using a start clutch. Due to engine downsizing and boosting for a given vehicle size, boost is not present at launch causing potentially insufficient transmission gear ratio for launching.
A powershift transmission generally has a specific number of gears and provides little design flexibility for accommodating an increase in the number of gears to five, six or seven speeds.
A powershift transmission also has complex electro-hydraulic controls to accommodate required synchronizer states. Some designs have relied on multiplexing clutch controls with synchronizer control in an attempt to deduce cost, which results in reduced operating performance, such as longer shift period, loss of repeatable high quality shifts, and an increased number of failure states.